This invention relates to the production of beams of energetic atoms, and, more particularly, to such production by forming and then disintegrating an energetic cluster beam.
The deposition of thin films upon substrates is an important manufacturing and research tool in a variety of fields. For example, microelectronic devices are prepared by depositing successive film layers onto a substrate to obtain specific electronic properties of the resulting structure. Photosensitive devices such as vidicons and solar cells are manufactured by depositing films of photosensitive materials onto substrates. Optical properties of lenses are improved by depositing films onto their surfaces. These examples are, of course, only illustrative of the thousands of applications of thin film deposition techniques.
In the highly controlled approach to thin-film deposition that is characteristic of applications wherein a high quality film is required, the film is built up by successive deposition of monolayers of the film, each layer being one atom thick. The mechanics of the deposition process can best be considered in atomistic terms. Generally, in such a process the surface of the substrate must be carefully cleaned, since minor contaminant masses or even contaminant atoms can significantly impede the deposition of the required highly perfect film. The material of the film is then deposited by one of many techniques developed for various applications, such as vapor deposition, sputtering, chemical vapor deposition, or electron beam evaporation.
In these and other atomistic techniques, matter and energy are supplied to the surface by particle beams, which can include ions, atoms, clusters, or some combination thereof. The particles in the beam may deposit upon the surface of the target, causing the gradual, layer-by-layer increase in thickness. The particles in the beam may also provide energy to the surface, and a carefully controlled energetic contribution has been shown to improve the perfection of the growing film.
The production of well controlled, energetic ion, ionized cluster and atom beams is therefore a key part of such processes. A number of techniques are known for producing ion and ionized cluster beams. The energy of the ions and ionized clusters of such beams is controllable to a good degree of accuracy, so that the particles in the beam have a narrow distribution in energies about a central value. Such control is possible largely because ionized particles react in a known way with electrostatic fields produced by accelerating electrodes provided within the deposition apparatus, and the magnitude of the field intensity governs the energy of the particles in the beam.
The production of well controlled, energetic atom beams is more difficult. Atoms do not interact with electrostatic fields in the apparatus, and therefore cannot be acceler158 the same manner as can ionized particles. In the normal practice for providing an atom beam, the atom particles in the beam must be produced and accelerated to a desired energy level by some physical means. As an example, a physical vapor deposition source produces a beam of atoms from a hot mass of the material to be vaporized, and the beam escapes from the source through an aperture. These atoms are not very energetic, typically with energies of about 1/10 electron volt per atom, and the energy cannot be readily increased by manipulation of the beam. In another approach, a beam can be produced by igniting a plasma that expands and imparts kinetic energy to atoms, propelling them toward a target. In this case the process is not well controlled, and there can be a wide range of energies in the atoms of the beam.
It would be desirable to have an approach for producing an energetic, bright beam of atoms wherein the atoms have energies on the order of 1 to 10 electron volts per atom. A source of such a beam would be useful in a wide variety of areas related to production of thin films, and also possibly in space propulsion and fusion reactors. The present invention fulfills this need, and further provides related advantages.